The invention relates to a system and method for converting digital component video signals, including a luminance signal and two-color difference signals having sample rates that are incompatible with the Phase Alternate Line (PAL) standard, into a digital composite video signal compatible with the PAL standard.
As used herein, the term "incompatible sampling rate" describes a sampling frequency which has no common multiple or only a relatively large common multiple with 17.734475 MHz, which is a commonly used sampling frequency for PAL signals. This frequency is four times F.sub.p, the PAL color subcarrier frequency.
Typically, when converting sampled data digital component video signals to a sampled data composite video PAL signal which has an incompatible sample rate, the sampled data digital component video signals are converted to analog signals, processed to generate an analog composite PAL signal, and then the analog composite PAL signal is sampled to generate a digital composite PAL signal.
The processing which generates the analog composite PAL signal generally begins by bandlimiting the analog signals. The bandlimited analog color difference signals are then used to modulate two quadrature phase related subcarrier signals which are added to the bandlimited analog luminance signal to generate an analog composite PAL signal. As stated above, the analog composite PAL signal is then sampled to form a digital composite
signal.
This processing system and method introduces sampling quantization errors during the analog to digital, and digital to analog conversions, and requires a substantial amount of processing.
The present invention performs the conversion process entirely in the digital domain using a method that requires fewer operations and so significantly reduces any errors. The digital composite video signals generated by this invention also can be converted to analog signals if desired.
As way of background, an analog PAL signal consists of a luminance signal, denoted as Y, added to a chrominance signal generated by modulating two quadrature-phase related sinusoidal signals at a frequency f.sub.p by two color-difference signals, denoted as U and V.